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At least 50 years prior to the battle, the Contender-class Artificial Intelligence032 Mendicant Bias had been assigned to a mission to locate and attack the FloodGravemind. As part of this mission, he was given control of a Halo Installation and charged with conducting the first test of such a weapon. After the test, however, Bias disappeared with the ring and was not seen for decades.

Bias unexpectedly returned with the Halo to the Forerunner Capital after 43 years, turned rampant by the Gravemind and bent on the destruction of the Forerunner race. He infiltrated the Capital's defense systems, deactivated all Ancillas and security constructs, and held the Council hostage by overriding their armor and rendering them immobile.

Bias was temporarily deactivated by Bornstellar-Makes-Eternal-Lasting, who was host to the memories of the Ur-Didact and issued a verbal failsafe code. The Capital was released from Mendicant's control, and began activating its defensive systems.

Despite losing control of the Capital, Bias could still control five of the twelve Halos in orbit around the capital. These rings immediately began charging their main weapons and positioning themselves around the capital in preparation to attack.

With the capital still recovering and the metarchy collapsed, Forerunner military responded to the threat with lethal force. All ships in the area began attack runs on the stolen Halos. The Monitors in charge of the Halo Installations, programmed to respond to any threat to their installations regardless of their source, engaged in a counter-attack. One of the Halos was destroyed due the heavy bombardment of the Forerunner fleet and the tidal forces of the Capital.

The seven Halos still under Forerunner control were ordered to travel through a nearby Slipspace portal and return to the Ark. One of them successfully made it through the portal intact, but was severely damaged by the stress of the transition. The second in line was destroyed upon the arrival of a third fortress ship, just as it began to enter the portal.

The outcome of the battle is largely unknown, as little information exists on events following Bornstellar's escape. It is known that at least one of the five Halos under Mendicant Bias' control was destroyed, but it is unknown whether any of the remaining four were successfully fired. Of the seven Halos that attempted to escape, only one successfully entered the portal. There it joined another array of installations under the control of the Librarian.